The present invention relates to a new and improved method of casting an article and more specifically to the method of casting a one-piece wheel having a hub portion and a plurality of airfoils which project outwardly from the hub portion.
One-piece metal wheels having a circular hub portion with radially outwardly projecting airfoils have previously been used in turbine engines. The airfoils on these wheels have previously been made with an equiaxed crystalline structure. Due to the severe operating conditions under which the one-piece wheels are used, it has been suggested that the wheels be formed with airfoils having an elongated columnar grain structure similar to that shown in U.S. Pat. Nos. 3,417,809 and 3,485,291.
In an effort to cast a one-piece wheel having airfoils with a columnar grain crystalline structure, it has been suggested that a mold be formed in the manner shown in U.S. Pat. No. 4,240,495. This mold is provided with a main section in which a circular wheel disc is cast and a plurality of radially outwardly projecting airfoil forming sections. Each airfoil forming section has an open inner end which is connected in fluid communication with the portion of the mold cavity in which the wheel disc is formed. The airfoil forming sections are closed at their outer ends where the tips of the airfoils are formed.
The aforementioned Patent No. 4,240,495 indicates that the desired columnar grain crystalline structure can be obtained by providing a chill formed of steel shot adjacent to the closed ends of the airfoil forming sections. This steel shot is described in the patent as providing a sufficient mass of thermally conductive material to withdraw heat from the airfoils and to promote the growth of columnar grains. According to the patent, the growth of the columnar grains is also promoted by the provision of cavities which are disposed adjacent to the airfoil forming portions. These cavities hold molten metal and are intended to prevent the airfoil forming portions from cooling too rapidly.
Since the mold and the shot chill shown in U.S. Pat. No. 4,240,495, are disposed in a container which is heated during preheating of the mold, the effectiveness of the chill is diminished. This impedes the formation of airfoils having a columnar grain structure with longitudinal axes of the grains extending parallel to the central axes of the airfoils. The formation of columnar grains which extend parallel to the central axes of the airfoils is further impeded by the necessity of solidifying the relatively thick leading edge portions and relatively thin trailing edge portions of the airfoils along fronts which extend perpendicular to the central axes of the airfoils.